This invention relates to a method of and apparatus for depositing amorphous carbon thin films that protect the magnetic media used in hard disk drives, to the media so formed, and to disk drives incorporating the media.
Metallic magnetic thin film disks used in memory applications typically comprise a substrate material that is coated with a magnetic alloy film that serves as the recording medium. Typically the recording medium used in such disks is a cobalt platinum-based alloy such as CoPtCr, CoPtCrTa, CoPtCrB, CoPtCrTaB or other various combinations of elements which is usually deposited by sputtering. The magnetic layer typically has an underlayer beneath it, consisting of Cr or Cr alloys, and also may have additional layers or seedlayers such as NiAl below the Cr layer. The substrates used for the deposited film are typically aluminum coated with plated NiP, or amorphous glass or glass ceramic materials.
Magnetic media is typically protected by a thin carbon overcoat film. An example of such a sputtered carbon overcoat is described by F. K. King in "Datapoint Thin Film Media", published in IEEE Transaction in Magnetics in July 1981, incorporated herein by reference. It is also known to provide a carbon film containing hydrogen by using hydrogen gas during sputtering as described in U.S. Pat. No. 5,045,165 by Yamashita. The protective overcoat is an extremely important component of thin film magnetic recording disks. When a disk drive is turned on or off, the recording head that normally flies over the recording surface comes into sliding contact with the media. Over the life of the disk drive, the head comes into many such "contact start-stop" with the media surface and the overcoat must protect the media from becoming destroyed in the process This is accomplished by providing a good wear resistant overcoat, and a thin layer of liquid lubricant that reduces the friction coefficient between the head and the media. Typically the media must withstand at least 20,000 cycles or more of repeated contact start-stop. It is also not unusual to have a specification of 100,000 contact start-stop capability for the disk and the disk drive.
Alternatively, the recording head can be pulled away from the disk surface when the drive is not in use or when it is turned off. In such drives, the contact start stop does not take place between the head and the disk surface. The designs that allow the head to be pulled to the side of the disk away from the disk surface are generally called ramp-loading mechanisms. However, the recording heads still fly very close to the media surface during operation of the drive, such that occasional contact occurs between the disk surface and the head. Therefore even in the drives using ramp-load mechanisms, the media must still have a protective overcoat.
As is known, magnetic films consisting of cobalt alloys are susceptible to corrosion during environmental extremes that the hard disk drives may see. The overcoat also protects the magnetic media from this corrosion. Today, the vast majority of the disks used for hard disk drives have sputtered carbon overcoat. Sputtering is a method of deposition whereby a target material to be deposited is "sputtered" or eroded by a plasma, and the plasma carries the sputtered species to the parts to be coated.
The thickness of carbon overcoat film has been steadily reduced over the years so that recording head can be made to fly closer to the magnetic media. As the active element in the head is placed closer to the magnetic layer, higher recording density can be achieved. Today, high performance hard disk drives have recording density in the range of 6-8 Gbits per square inch (about 1 Gbits per square centimeter). A media for such application has an overcoat thickness that is less than 10 nm thick and the recording heads fly at around 30 nm from the top of the carbon film. In order to increase the recording density even higher, there is a strong desire to reduce the carbon overcoat thickness further to 5 nm or even thinner, and also to reduce the flying height of the recording head below 25 nm. At the same time however, the media is expected to meet or exceed the reliability of current media having thicker carbon overcoat and higher flying heights. Typical measures of reliability are contact start stop capability and corrosion resistance. It has been very difficult to meet such expectations from a conventionally sputtered carbon overcoat as overcoat thickness is reduced below 10 nm.
A carbon film deposited by sputtering is amorphous. It has no evidence of crystallinity down to the resolution of the most powerful transmission electron microscopes. To enhance the properties of the sputtered carbon films in terms of contact start stop performance or corrosion resistance, additives such as hydrogen have been added to the film as described in U.S. Pat. No. 5,045,165. Alternatively nitrogen or a combination of nitrogen and hydrogen is also sometimes used to enhance contact start stop performance. The sputtering process typically involves applying DC power onto a graphite target. A type of sputtering cathode called DC magnetron cathodes are typically used. Depending upon the size and configuration of the cathode, typical voltages that develop on the target are between 300 to 800 volts negative DC with various current level to achieve the necessary deposition rate.
Alternatively, the DC power can be applied as a pulse, using power supplies available from companies such as Advanced Energy Industries, Inc. based in Fort Collins, Colo. for example. In such case the pulse rate can be fixed or adjustable to typically 20 kHz to 50 kHz range. Alternatively, an AC power supply can be used, or a combination of AC and DC power supply as described by Yamashita et al. in U.S. Pat. No. 5,507,930. Generally speaking, the carbon films sputtered by these methods have similar atomic structures. The carbon in these films have between 30 to 50% of their bonds in sp.sup.3 bonding configuration depending upon the method of measurement that is used. The typical film stress for these films are around 1 to 2 giga-pascal (GPa) compressive for a film of around 10 nm thickness.
The vast amount of scientific work on carbon films in recent years has caused investigators to adopt common conventions to describe the film types based on their chemical structure. For amorphous carbon films, the following conventions are in general use:
______________________________________ a-C amorphous carbon a-C:H hydrogenated amorphous carbon ta-C tetrahedral amorphous carbon ta-C:H tetrahedral hydrogenated amorphous carbon ______________________________________
The amorphous carbon film should be distinguished from actual diamond film which is composed of crystalline diamond and has a high sp.sup.3 bonding fraction. Both sputtering and chemical vapor deposition methods such as plasma-enhanced chemical vapor deposition (PE-CVD) and ion-beam deposition produce largely amorphous carbon films. Hydrogen is almost always present to varying degree from the hydrocarbon gas used in the CVD type process, and in the case of sputtering, from deliberate introduction of hydrogen in the plasma. Amorphous carbon films containing a significant amount of hydrogen are categorized as "hydrogenated amorphous carbon films" and designated as a-C:H films.
The vast majority of sputtered carbon overcoat typically used in the magnetic media can be categorized as a-C:H type carbon film, due to incorporation of 20 to 40% atomic fraction of hydrogen in the film. A designation as an a-C or a-C:H carbon film also implies that the fraction of its bond in the sp.sup.3 or tetrahedral bonding configuration is relatively low, typically from around 10 to 50%. It is known that hydrogen incorporation increases sp.sup.3 content of sputtered carbon films to a maximum of around 50% by current knowledge.
In order to enhance the properties of the carbon films, various investigators have been recently focused on creating a new high sp.sup.3 content amorphous film designated as ta-C or ta-C:H carbon film, which stands for tetrahedral amorphous carbon film, indicating a high fraction of bonding in the sp.sup.3 state. In order to qualify as ta-C carbon, the sp.sup.3 bonding fraction in the film should be at least 70% a n d preferably in the range of 80% or higher. Another term often used for the ta-C carbon is "DLC" or diamond-like carbon. However, the term "DLC-carbon" has been widely misused so that it may be best not to use this terminology for the amorphous carbon film. The main reason for the confusion in terminology for and description of the film has been the lack of clear measurement tools that definitively define the properties of the carbon film in the past. Currently, the situation has been greatly improved due to better understanding of the tools and the measurements themselves, and due to the application of new tools to enhance the understanding of the nature of the films.
ta-C or ta-C:H carbon films have superior mechanical and corrosion properties compared to the a-C:H carbon by virtue of the high sp.sup.3 content. It is known to have higher compressive stress, which leads also to a higher hardness in the film. The density is also known to be higher than a-C or a-C:H film, although the density can vary as function of hydrogen content as well. For magnetic hard disk application, it is thought that ta-C:H carbon offers much better properties in terms of contact start-stop and corrosion resistance especially for films less than 10 nm in thickness. There is considerable desire to reduce the film thickness to around 5 nm currently, and even down to 2 nm in the near future. However, these thinner films must not lose any performance compared to current films. Since it is also believed that conventional sputtering method cannot create ta-C:H type carbon film, recent efforts to produce such carbon have focused on the use of PE-CVD or ion-beam deposition method.
PE-CVD and ion-beam depositions are fundamentally different methods of deposition compared to sputtering. The source of carbon in sputtering is typically a graphite target, while in PE-CVD and in ion-beam deposition, the source is hydrocarbon gasses. Both PE-CVD and ion-beam deposition can be categorized as forms of CVD, since both use hydrocarbon gas to create the carbon ions that are used to form the film. A more detailed description of the ion-beam method of forming the carbon film is described by Weiler et al. in "Preparation and Properties of Highly Tetrahedral Hydrogenated Amorphous Carbon" published in Physical Review B, January 1996. In a narrow range of deposition condition and energies of carbon ions, Weiler et al. report on fabricating ta-C:H film by their form of ion-beam deposition.
The benefits of ta-C:H carbon is described by Weiler et al. in the above mentioned article. The sp.sup.3 bonding fraction in ta-C:H carbon is as high as 80%, and the film stress is much higher for a given thickness of the film compared to films with lower sp.sup.3 content. Consequently the density and hardness of the film is highest at the highest sp.sup.3 content. However, in PE-CVD or ion-beam deposition, the process must be tuned fairly precisely, to maintain the energy per carbon atom ion near 100 ev/atom for their method. Otherwise the sp.sup.3 fraction drops precipitously on either side of the energy value. The films produced according to Weiler's prescription for ta-C:H carbon film may have several advantageous properties for hard disk application. At 5 nm film thickness for example, the film known to contain high sp.sup.3 fraction made by ion-beam and PE-CVD method continues to exhibit good contact start-stop performance and corrosion resistance, while conventionally sputtered a-C:H films starts to show a deterioration in performance.
Both PE-CVD and ion-beam deposition methods have one common drawback, which is that they create a tremendous amount of flaking and particles during deposition compared to sputtering. The hard disk media must be maintained extremely clean. Even a presence of small amount of particles that can be trapped between the disk and the head can cause catastrophic head crashes since the head flies very close to the surface. This difficulty has prevented more widespread use of PE-CVD and ion-beam deposition in hard disk manufacturing. In deposition applications that require only intermittent use of the tool such as in semiconductor and recording head manufacturing, both PE-CVD and ion-beam methods have found more widespread use. In hard disk manufacturing however, the manufacturing tool must run continuously for at least 5 days or more in order for it to be economical. For example, the in-line sputtering systems as described in U.S. Pat. No. 5,045,165 by Yamashita can run continuously for 4 weeks without opening the system for servicing. It has been difficult to obtain even a few days of continuous deposition out of the CVD or ion-beam deposition tools due to particle generation during deposition. Such short utilization severely impacts the productivity of the expensive deposition tools, and increases the cost of manufacturing. In addition, the cathodes used for PE-CVD and ion-beam deposition are also expensive compared to comparable sputtering cathodes.
In some implementations of PE-CVD and ion-beam deposition methods, substrate bias is required in order to obtain the ta-C:H type carbon films. It is more advantageous to have a process which does not require substrate bias since some substrates that are used for hard disk application are insulators, such as glass and glass ceramics. It is necessary to have conductive substrates in order to apply substrate bias to the disk. For ion-beam deposition, many cathode designs have problems that impact their usefulness in continuous deposition tools. In some designs, a hot filament is needed in front of the cathode as source of electrons to energize the plasma. The filament can burn out and can be coated with the film of the material to be coated and cause significant reduction in the amount of time that the cathode can be operated. In other designs, the potential that is needed to extract the beam of carbon ions is applied through a thin grid placed in front of the gun. These grids wear out due to bombardment by the ions, or else are coated by the film to be deposited, and therefore must be periodically replaced. The grid also contributes significantly to particle generation as it is rapidly coated during the deposition and begins to flake.
Finally further difficulties involved in PE-CVD or ion-beam deposition methods is that they require hydrocarbon gasses as source of carbon ions. Typical gases are acetylene, ethylene, naphthalene and butane. These gases are hazardous and require special precautions. Special handling and safety requirements must be rigorously followed for safe operation. Typically, large capital investments are needed to meet regulatory requirements. Related to the use of hydrocarbon gas is that film uniformity is often difficult to achieve in PE-CVD and ion-beam deposition. The film uniformity depends sensitively on hydrocarbon gas uniformity within the cathode, which is often difficult to control. Although the uniformity is in principle controllable, it has been more difficult compared to the conventional sputtering method, which is more established.
Manufacturing of hard disk media is typically done in two different type of machines. The first is called "in-line" deposition tool, and it is described in U.S. Pat. No. 5,045,165 by Yamashita. In such a tool, the disks are loaded on large pallets which pass by series of cathodes which deposit the films successively on a disk. Alternatively, the disk can be deposited one disk at a time using circular targets or cathodes using a machine tool generally called a "static" deposition tool. Such machines are manufactured by Intevac Inc. of Santa Clara, Calif., Balzers Process Systems, Inc. of Alzenau, Germany and Anelva Corporation of Fuchu-shi, Japan.
To increase productivity and reduce the cost of manufacturing, it is important to operate the sputtering machine continuously with high utilization. There are several factors which limit the continuous operation of the sputter machines. First, the target can be used up, and the system must be opened to replace it. Second, the protective shields that direct the sputtered species to the substrate become coated, and they will eventually start to flake. The shields must be periodically replaced and cleaned. Excessive flaking from the shields contributes to increased defects on the disk surface.
A third factor that is specific to carbon sputtering is that the carbon target has finite run times due to formation of defects on the target surface, which are generally called "nodules," "warts" or "mushrooms" which degrade the performance of the cathode. Formation of these defects on the target surface contributes to an increased rate of arcing on the target. Arcs cause defects and particles to be deposited on to the disk surface. An increasing rate of these defects can cause premature shut-down of the system. During a shut down, the sputtering system is vented to air and the targets are cleaned or replaced. The time needed to do the maintenance and target changes contributes to a loss of utilization and adds cost to manufacturing. Typically the sputter line is the most expensive capital equipment needed in making the disks, therefore a loss in utilization impacts seriously the overall cost of manufacturing.
A method described by Yamashita et al. in U.S. Pat. No. 5,507,930 teaches one method of reducing nodules on the carbon target surface by use of AC power superimposed on DC which also reduces arcing. Even with such methods however, the carbon target is still susceptible to nodule formation and excessive arcing. Therefore conventional sputtering is still far from being ideal, but it is still more suitable for manufacturing than PE-CVD and ion-beam deposition methods. An additional method to reduce nodule formation has been to use a pulsed DC power supply which provides a square wave type positive voltage pulse at a frequency of 20 kHz for a duration (pulse width) of several micro-seconds (.mu.-seconds).